This project was initiated to fill a void in our knowledge regarding the neurobiological substrates of the the delterious effects of chronic marijuana use on cognition in humans. It is well-known that both acute and chronic marijuana use in humans impairs short-term memory, reaction times, and general higher-order cognitive processing. These studies seek to utilize an animal model to explore the effects of chronic exposure to the main psychoactive ingredient in marijuana, delta9-tetrahydrocannabinol (THC) on the neurophysiology of the hippocampus. The experiments involve repeated i.p. injections with THC for varying periods of time, followed by varying periods of withdrawal from the drug, and then evaluation of electrophysiological parameters in brain slices containing the rodent hippocampus. We will also utilize genetically modified strains of mice in which the cannabinoid CB1 receptor has been invalidated (CB1-/-), and compare effects observed in these mice to those observed in wild type littermated (CB1+/+). Our first studies are aimed at defining the sites of cannabinoid action in the hippocampi of these animals, and exploring the effects of chronic THC on both CB1 and a novel cannabinoid receptor we have defined as "CBsc". This non-CB1 cannabinoid receptor was previously defined by the persistence of inhibition of glutamatergic EPSPs by the cannabinoid receptor agonists in CB1-/- mice, and by its sensitivity to the antagonist SR141716A and its insensitivity to the antagonist AM251. We have examined CBsc receptors in Sprague Dawley (SD) rats, and two strains of wild-type (WT) mice (C57BL/6J and CD1) used as backgrounds for 2 independent lines of CB1-/- mice. The inhibition of synaptic glutamate release by the agonist WIN55,212-2 was observed in hippocampal slices from WT CD1 mice, and SD rats, but was absent in WT C57BL/6J mice. The results indicate that the glutamate-modulating CBsc cannabinoid receptor is present in the hippocampi of CD1 mice, and SD rats, but not in C57BL/6 mice. Thus, we have identified animal models that may permit the study of cannabinoids independently of the novel CBsc receptor (C57CB1+/+), the CBsc receptor independently of the cloned CB1 receptor (CD1CB1-/-), or in the absence of both receptors (C57CB1-/-). Additional studies now in progress indicate that following a 1d withdrawal from a 7d treatment with THC, the drug was undetectable in hippocampus using GC-MS. However, the 7d exposure to THC produced tolerance to the inhibition of GABA, but not glutamate release by the cannabinoid agonist WIN55,212-2. Additionally, unlike controls, the hippocampal slices from the chronic THC animals did not demonstrate long-term potentiation (LTP), a cellular model of learning. A single injection of THC was insufficient to block LTP, the LTP blockade persisted for 3d after the last THC injection, and it was prevented by pretreatment of the animals before each THC injection with the antagonist AM251 (2 mg/kg). To determine the sites at which THC acted to disrupt LTP, experiments were conducted in CB1-/- mice. Compared to rat, LTP was only partly blocked by 7d THC in the CB1-/- mouse. These experiments will define the consequences of repeated THC exposure in the rodent hippocampus, and identify putative molecular tar